This invention relates to photovoltaic devices, and more particularly to an induced junction semiconductor structure for use as a solar cell.
There is a continuing need for solar cells of high efficiency and reliability to power electronic systems in space vehicles, and an emerging need for high-efficiency solar cells for use on Earth in place of more conventional sources of energy. The most efficient solar cell which has been produced is basically comprised of a diffused silicon p-n junction. Since Chapin, Fuller and Pearson first reported the development of such a solar cell in "A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power," J. Appl. Phys. Vol. 25, 676 (1954), other semiconductor materials, such as GaAs and In, P, have been used. However, silicon is perhaps the most widely used because of the high efficiencies achieved and the ease with which silicon devices can be produced even though other materials have higher predicted efficiencies at different temperatures from below room temperature to as high as 400.degree. C.
The operation of a diffused p-n junction solar cell is based upon hole-electron pair generation by photons and migration of minority carriers on each side to the other side (i.e., migration of electrons from the p side to the n side and holes from the n side to the p side.) Recombination of hole-electron pairs in the depletion region of the p-n junction reduces the efficiency of the solar cell to a range of about 10% to 15%, about half the efficiency that is theoretically possible.
Professor R. L. Call of the University of Arizona in Tucson, Arizona, has proposed an induced junction at the surface of a silicon wafer for use as a solar cell. The structure fabricated and tested consisted of p-type silicon in which two n-diffused strips were employed as contacts to an inversion region between them. The inversion region in the silicon was achieved by a transparent sheet electrode deposited over a thermally grown oxide layer. The sheet overlaps the diffused stripes to assure an inversion region from one stripe to the other. Aluminum contacts are evaporated over the stripes in order to connect a load to the cell. A positive potential applied to the transparent electrode induces an inversion layer near the surface of the silicon wafer. The inversion layer induces a junction, and the result is a metal-insulator-silicon (MIS) diode which then operates like a p-n junction diode as a solar cell.
As an alternative to the transparent conductive film over the oxide insulator used as an electrode to apply an electric field, Professor Call has suggested purposely placing ions from contaminants in the oxide insulator, such as ionized sodium. Increasing the thickness of the oxide layer, with uniform distribution of ions, increases the total charge density and therefore increases the inversion layer. However, using dopants like sodium are unsatisfactory because the charge distribution is unstable at operating temperatures.